The present invention relates to a device for arresting the propagation of a buckle in a double-walled pipe which can be wound onto a reel and more particularly in a rigid pipe used for transporting fluids such as hydrocarbons.
A rigid pipe or tube is laid on the seabed usually from a so-called pipelaying vessel. The laying is called S-laying when the pipe adopts the shape of an S between the pipelaying vessel and the seabed and it is called J-laying when the pipe adopts the shape of a J. In the latter case, a guide ramp is provided on the pipelaying vessel, which ramp may sometimes be partially immersed in the water.
The rigid pipe to be laid is stored on the pipelaying vessel either in pipe sections of a given but relatively short length, the pipe sections being joined together as the laying progresses, or it is wound as a great length on a reel, the pipe then being unreeled from the said reel during the laying operation. These laying operations are described in the API (American Petroleum Institute) document “Recommended Practice 17 A” from 1987.
When the pipe has left the vessel and while the pipe is being laid, it is important that the pipe undergoes no plastic deformation in bending, which would result in ovalization of the pipe, as ovalization causes a “singular weak point” which would be conducive to the initiation of a collapse. Moreover, when the pipe is laid on the seabed at great water depths (typically greater than 300 m and possibly up to 2000 m and more), the hydrostatic pressure exerted on the pipe may be sufficient to initiate a buckle which has a tendency to propagate along the pipe, in both directions. Of course, the buckle will form preferentially at a “singular weak point” when one exists on the pipe. When the buckle occurs, it is then necessary to replace at least that section or portion of the pipe comprising the buckled or collapsed region.
To prevent the propagation of a local buckle or buckles, it has been proposed to provide the pipe with certain devices or means, called buckle arrestors.
Such buckle arrestors are described in the U.S. Pat. Nos. 2,425,800, 3,747,356, 3,768,269 and 4,364,692.
The process in U.S. Pat. No. 3,747,356 consists in linking a cylinder to a cable, in lodging the cylinder inside a pipe section and then in simultaneously unreeling the pipe and the cable so as to keep the cylinder in the pipe section while the latter is being laid, until the pipe comes into contact with the seabed. The cylinder is then brought back up so as to be lodged in another pipe section to be laid, which is joined to the previous section. Consequently, any buckle likely to occur, when laying the pipe, between the pipelaying vessel and the seabed is immediately arrested and therefore not allowed to propagate along the pipe sections. However, such an arrangement provides no solution or effectiveness for arresting buckles likely to be propagated after the pipe has been finally laid on the seabed.
In U.S. Pat. No. 3,768,269, it is proposed to locally increase the stiffness of the pipe by placing, at regular intervals, for example at intervals ranging between 100 m and 500 m, reinforcing collars whose length ranges between 1 m and 2.5 m. Such a solution is valid only for pipes laid in sections since the reinforcing collars can be mounted and fastened in the factory to the pipe sections and then transported by the pipelaying vessel to the laying site. When the pipe is long and wound onto a storage reel, it then becomes virtually impossible to wind the pipe with its reinforcing collars onto a reel since they would result in straight or almost straight portions that cannot be deformed when winding the pipe onto the storage reel. In order to mitigate this difficulty, it is conceivable to mount and fasten the reinforcing collars during the laying operations. However, it would then be necessary to interrupt the laying, at regular intervals, so as to mount and fasten the reinforcing collars.
In order to allow the pipe to be wound onto a reel, U.S. Pat. No. 4,364,692 proposes to wind a rod tightly around the pipe so as to form a certain number of turns which can be welded at their ends to the rod itself and/or to the pipe.
According to another embodiment, the turns may be individual turns, by welding their two ends and regularly spacing them apart along that portion of the pipe to be reinforced. As long as the pipe is a single-walled pipe, the increase in the diameter in the reinforced portions may be acceptable. However, when the pipe is of the double-walled or pipe-in-pipe type, that is to say comprising an external pipe or carrier pipe which is slipped over the internal pipe, the increase in the diameter of the carrier pipe is unacceptable when transporting and storing long lengths of pipe of the pipe-in-pipe type.
When the rigid pipe to be laid is manufactured in long lengths on land and then wound onto a reel on the pipelaying vessel, the solutions recommended in the aforementioned documents are not appropriate as they use either long reinforcing collars, having a length of about 1 to 2.5 m, as in U.S. Pat. No. 3,768,269, or the winding of a reinforcing rod around the rigid pipe, as in U.S. Pat. No. 4,364,692.
Another propagation arrestor is described in U.S. Pat. No. 3,860,039 and consists in placing a sleeve over a rigid pipe so as to have a constant outside diameter with a liner. The annular space between the sleeve and the pipe is filled with tar, which transfers the collapse force to the sleeve.
Other end-blocking systems, or bulkheads for double-walled rigid pipes exist and are described especially in WO 96/36831 and WO 98/17940. Such bulkheads cannot be likened to propagation arrestors since the material from which they are produced is not capable of transferring the stresses applied to the carrier pipe on the internal pipe.